USB peripheral prevents boot

So I'm trying to install bootcamp with an iso image on a USB drive. I understand the boot camp process and have completed it before without a USB, but that's the only option available to me now. I have the gray screen problem whenever I try to restart my MBP Retina late 2012 with any non-native peripherals attached - it simply delays at the gray screen until I disconnect the USB and force restart my laptop. You can probably imagine what I'm going to say next.

When trying to install windows through the Boot Camp application, on the step where I restart after partitioning to boot to windows, the USB prevents the computer from getting past the gray screen. If I disconnect and force restart by holding down the power button, a Windows command prompt-esque interface will come up that says "No bootable disk - insert bootable disk and press any key" or something along those lines. It's clear the start up disk is windows, and it will continue to boot to that screen unless I option key-out of it on a restart. If I try connecting my USB once the command prompt screen has come up, it simply won't read it. I've left it in for about 5 minutes and tried basically every key to try to get it to work, to no avail.

I'm wondering if there's any way to fix this issue or work around and install windows with a USB still. Any help is much appreciated, thanks.